rgw: add pipe fd to set for select() in do_curl_wait()

rgw: add pipe fd to set for select() in do_curl_wait()

when HAVE_CURL_MULTI_WAIT is 0, the pipe fd is never added to the
readfds for select(), so FD_ISSET() is always false. this prevents us
from ever trying to read from the fd, and the pipe's buffer eventually
fills up and deadlocks callers of RGWHTTPManager::signal_thread() when
they try to write to the pipe

Fixes: http://tracker.ceph.com/issues/16368
Signed-off-by: Casey Bodley <cbodley@redhat.com>